A downhole steering tool (or “controllable stabiliser”) is described in EP 1 024 245. As indicated in that document, the steering tool is used to control the drilling direction by forcing a part of the drillstring away from the longitudinal centreline of the borehole, thereby forcing the drill bit to deviate from a linear path. The steering tool of EP 1 024 245 carries a drive shaft which is connected to the drillstring and rotates with the drillstring, the drive shaft being surrounded by the annular steering tool. The steering tool has actuators which can move a part of the tool radially, and thereby move the drive shaft and drillstring away from the longitudinal centreline of the borehole.
In order to steer the drill bit in a chosen direction it is desirable that the steering tool does not rotate with the drillstring but instead only moves longitudinally along the borehole as the drill bit advances, maintaining a chosen orientation or azimuth within the borehole. However, there is a tendency for the steering tool to rotate with the drive shaft. It is therefore known to provide a braking mechanism which can engage the borehole wall in order to reduce the rotation of the steering tool.
It is recognised that the braking mechanism will not always prevent rotation of the steering tool within the borehole, and the tool contains sensors to detect its actual azimuth (as well as its inclination and toolface) so as to ensure accurate control of the steering direction. Nevertheless, it is intended that the braking mechanism significantly reduces the rate of rotation of the steering tool.
Ideally, the braking mechanism should provide a minimum resistance to the continued advance of the steering tool and therefore the drill bit during the drilling operation, and should provide a minimum resistance to the longitudinal movement of the steering tool during tripping of the downhole assembly.
A known braking mechanism used on practical embodiments of the steering tool of EP 1 024 245 includes a torsion beam and is often referred to as an “anti-rotation lever”. The braking mechanism has a braking member in the form of a metal blade which is designed to engage the borehole wall, the blade lying in a plane which is substantially radial to the steering tool and substantially parallel to the longitudinal axis of the steering tool. When the blade engages the borehole wall (and perhaps digs into the borehole wall), it provides minimum resistance to longitudinal movement of the steering tool, but provides maximum resistance to rotation of the steering tool (around the longitudinal axis of the steering tool).
The blade is mounted at one end of the torsion beam, the other end of which is securely fixed to the steering tool. The torsion beam provides a flexible but resilient connection between the steering tool and the blade, and in its rest condition causes the blade to project beyond the periphery of the steering tool. In practical applications there are typically three or six braking mechanisms spaced approximately 120° or approximately 60° apart, respectively, around the circumference of the steering tool, each of the blades being biased to project a predetermined distance (usually a few millimeters) beyond the tool periphery. It is arranged that the drill bit produces a borehole which is nominally only slightly larger than the steering tool, i.e. the radial gap between the borehole wall and the steering tool is substantially less than the radial protrusion of the blades in their rest condition, so that in use the blades adopt a nominal position in which they are substantially continuously pressed against the borehole wall as the drill bit advances.
It is a known problem with downhole tools that the radial distance within which componentry can be located is relatively small. Thus, all downhole tools must accommodate a bore through which drilling mud can be communicated to the drill bit, and at least one peripheral channel through which drilling mud and entrained drill cuttings can be returned to the surface. A steering tool such as EP 1 024 245 in particular presents significant space limitations in that the tool must have a large enough bore to accommodate the drive shaft (with its own bore for the drilling mud), as well as all of the componentry needed to control and move the actuators. In practice, the radial size of the steering tool within which the braking mechanism and other componentry can be mounted is often only a few centimeters.
Another braking mechanism for a downhole steering tool (referred to as an “anti-rotation device”) is described in U.S. Pat. No. 7,306,058. The first embodiment of this document discloses a number of rollers mounted upon a carriage, the axis of rotation of the rollers being tangential to the steering tool so that the rollers can roll along the borehole wall as the drill bit advances, and resist rotational movement of the steering tool. The carriage is biased outwardly of the steering tool by a set of coil springs.
The second embodiment uses a number of pistons which are driven outwardly of the steering tool (and into engagement with the borehole wall) by coil springs acting in cooperation with pneumatic or hydraulic pressure acting underneath the pistons.
The downhole environment in which a steering tool must operate is extremely harsh, including substantial pressures and very high temperatures. There is therefore an advantage in providing a braking mechanism which is mechanically simple and is therefore less likely to fail in use. Mechanically simple braking mechanisms such as the torsion beam described above are therefore in widespread use.
The force which can be provided by the known braking mechanisms is significant. The torsion beams used in the steering tool of EP 1 024 245 for example are designed to exert a force of 600-700 N on the borehole wall. Such forces contribute towards the harsh environment and can cause damage to the steering tool in the event that the borehole wall contains discontinuities such as voids which can cause a rapid change in the diameter of the borehole (and therefore a rapid extension and subsequent retraction of the blades). Since the borehole wall will seldom be smooth and of uniform diameter throughout its length, the steering tool must be able to accommodate the mechanical shocks caused by rapid movements of the blades as they move along the borehole wall.
In addition to their mechanical simplicity, torsion beam braking mechanisms require relatively little radial space in which to operate, and are therefore useful in a steering tool where the radial space is limited. However, they have the disadvantage that they require significant longitudinal space, i.e. the torsion beam is relatively long. This places limitations upon the location of the braking mechanism upon the steering tool. In addition, since the braking mechanism must be positioned upon a linear section of the steering tool, the length of the torsion beam contributes towards the minimum possible length of the steering tool. It will be understood that a long steering tool cannot pass along a sharply curved section of borehole, and steering tool designers are seeking to provide shorter steering tools which place a higher limit upon the possible borehole curvature.